1. Field of the Invention
The invention relates to a steerable stylet for a medical implantable lead of the type having an inner lumen, into which the steerable stylet is insertable for navigating a distal end of the medical implantable lead to the desired location for attachment to tissue, the steerable stylet comprising an elastic tube, a likewise elastic wire, being insertable into the tube, and an actuator in a proximal end for manipulating them in relation to each other, wherein the tube and the wire being arranged such that, when the wire is inserted into the tube, the tube and the wire are non-rotatable in relation to each other and at least the wire is pre-bent in one direction such that a distal portion of the steerable stylet, and hence also the medical implantable lead, may be bent in desirable degree by displacing the tube and the wire in relation to each other such that a portion of the wire protrudes from the distal end of the tube and, when the medical implantable lead is bent, the distal portion of the medical implantable lead is pivotal by means of the actuator to which the proximal end of the wire is non-rotatably connected.
The invention also relates to a method for manufacturing of a steerable stylet for a medical implantable lead.
2. Description of the Prior Art
A steerable stylet of the above kind is known since long time and is used to navigate the distal end portion of a medical implantable lead inside a human or animal body to locate the distal end tip of the lead to a desired position in the tissue. For instance it is common practice to use such a steerable stylet to navigate the distal end tip of a pacemaker lead inside a heart to attach the lead tip to an appropriate position at the heart wall. However, the steerable stylet could also be used to navigate other types of medical implantable leads for monitoring, measuring, controlling or treating arbitrary organs inside the body.
The tube and the wire of the steerable stylet are displaceable in relation to each other. However, the tube and the wire are also unrotatable arranged in relation to each other. This can be accomplished in different ways, but normally it is achieved by that a portion of the inner bore of the tube being non-circular in cross section and the wire being in cross section complementary to the tube bore section such that they can be displaced but not rotated in relation to each other. This portion of the tube and the wire is preferably located in their distal end portions but could also be located on a distance, e.g. 10 to 15 cm, from the distal end. By also manufacturing the tube as well as the wire of an elastic material and pre-bend at least the distal end portion of the wire in one direction or, preferably, pre-bend the distal end portions of both the tube and the wire such that they are pre-bent in opposite directions when assembled together, the tip portion will be satisfactory straight when the ends of the tube and the wire are in an initial position, preferably with their ends coinciding with each other, in which case the straight tube, which has the largest flexural rigidity, will counteract the bending force in the wire, or the bending forces in opposite directions of the tube and the wire will essentially counterbalance each other, whereas the tip portion will bend in one direction when the tube is retracted in relation to the wire, or the wire is advanced in relation to the tube. Since the steerable stylet is positioned inside the lead, which is highly flexible, also the tip portion of the lead will bend in the same direction. The degree of curvature can be regulated by changing the amount of retracting or advancing of the tube and the wire, respectively. With the tip portion in a curved state, it can subsequently be pivoted by rotating the actuator to seek the best possible position for the tip portion. The navigating procedure can be visually monitored by means of e.g. x-ray imaging or magnetic resonance imaging.
In prior art technique, it is common practice to have only the wire unrotatably connected to the actuator, which has to result that only the wire will contribute to the pivoting motion of the lead tip. This is done for security reasons, since if the lead tip should be constrained from pivoting, e.g. due to abutment against tissue or the like, and the steerable stylet is overstressed by excess rotating of the actuator, the steerable stylet will eventually failure by way of the wire most likely snapping off at the attachment point to the actuator. This is harmless since the physician will be aware of the failure and the wire can easily be drawn out from the lead without risking that any part of it will be left in the lead or the body. However, when only the wire is contributing to the pivotal movement, the movement will be difficult to control since the wire is so weak and has a low torsional stiffness. Hence, it is difficult to navigate the lead tip to the desired position. This problem could be overcome by attaching also the tube non-rotatably to the actuator. However, this could be risky since then overstressing of the steerable stylet could lead to that the wire, which most likely will failure before the tube since it is much weaker, could snap off at a region at its distal end portion since in this case the strains will be as largest in this region. A failure in this region will be more dangerous since here the failure is harder to recognize and pieces from the wire—or the tube—can be left in the lead or the body.